Myostatin and TGF‐β signaling in overload‐induced skeletal muscle hypertrophy (1163.14)

Abstract

Myostatin (GDF‐8) negatively controls adult skeletal muscle growth. The role of myostatin and TGFB signaling pathways in overload‐induced hypertrophy of adult muscle is unclear. We tested the hypothesis that overloaded adult rodent skeletal muscle would reduce skeletal muscle myostatin levels and thereby lower pSMAD2/3 levels and increase pAKT levels. Overload‐induced hypertrophy was induced by unilateral tenotomy of the gastrocnemius tendon in 11 male adult Sprague‐Dawley rats followed by a 2‐week period of compensatory overload of the plantaris and soleus muscles. Western blot analysis was performed to evaluate changes in active and latent complex myostatin protein levels and the proteinase BMP1/TLD. The TGFβ signaling pathway was evaluated using a Milliplex magnetic bead multiplex assay (TGFβ receptor, pSMAD2, pSMAD3, SMAD4, pERK, and pAKT). Significant hypertrophy was noted in both the soleus (494 ± 29 vs 405 ± 15 mg, p<0.05) and plantaris (289 ± 12 vs 179 ± 37 mg, p<0.05) muscles following overload. Overloaded skeletal muscle reduced active myostatin protein (range 32.7 ± 9.4% to 28.5 ± 8.5%, p<0.05), latent myostatin protein (range 15.0 ± 5.9% to 70.0 ± 2.3%, p<0.05), and the BMP‐1/TLD proteinase (range 40.4 ± 12.9% to 32.9 ± 6.9%, p<0.01). Changes in TGFβ signaling were muscle type dependent. In the predominantly fast twitch plantaris muscle there was an increase in TGFβ‐r, pSMAD2, pSMAD3, SMAD4, and pAKT (p<0.05). In the predominantly slow twitch soleus muscle there was an increase in pERK, pSMAD2, and pSMAD3 (p<0.05). The paradoxical increase in pSMAD2/3 and pSMAD4 with a decrease in active myostatin in either muscle type may be related to cross talk between an ERK‐mediated phosphorylation of SMAD2/3 and/or other TGFβ stimulated MAPK pathways.